Preparation of New Nitrogen-Bridged Heterocycles. XXXI. Cyclization Aptitudes of 2-Acylmethylthio-3-cyanoindolizines Having a Cyano or an Ester Group at the 1-Position.
Preparation of New Nitrogen-Bridged Heterocycles. XXXI. Cyclization Aptitudes of 2-Acylmethylthio-3-cyanoindolizines Having a Cyano or an Ester Group at the 1-Position.
Some 2-acylmethylthio-3-cyanoindolizine derivatives having a cyano or an ethoxycarbonyl group at the 1-position were prepared and their intramolecular cyclization reactions were examined. The alkaline treatment of these polyfunctionalized indolizines in refluxing ethanol gave only 3-aminothieno[2, 3-b]indolizine derivatives in moderate to good yields regardless of the kind of 1-substituent, and the other possible alternative, a thieno[3, 2-α]indolizine derivative, could not be obtained at all. The higher reactivity of the 3-cyano group over 1-cyano and 1-ethoxycarbonyl groups coincided well with the results expected from molecular orbital calculations for the model compounds. From our present and previous results, the order (3-CN>1-CN>3-ester>1-ester) of the reactivities of electron-withdrawing groups at the 1- and 3-positions on the indolizing ring was finally estblished.